Recently, in line with the development of information and telecommunications industry, electronic devices are being miniaturized, light-weighted, reduced in thickness, and portable. As a result, the need for high energy density batteries used as power sources of such electronic devices has increased. Currently, research into lithium secondary batteries, as batteries that may best satisfy the above need, has been actively conducted.
A lithium secondary battery is a battery which is composed of a cathode, an anode, and an electrolyte and a separator which provide movement paths of lithium ions between the cathode and the anode, wherein electrical energy is generated by oxidation and reduction reactions that occur when lithium ions are stored in and released from the cathode and the anode.
A lithium secondary battery has an average discharge voltage of about 3.6 V to about 3.7 V, and one of the advantages of the lithium secondary battery is that it has a higher discharge voltage than other alkaline batteries and a nickel-cadmium battery. In order to achieve such a high operating voltage, an electrolyte composition, which is electrochemically stable in a charge and discharge voltage range of 0 V to 4.2 V, is required.
Lithium ions released from a cathode active material, such as lithium metal oxide, during initial charging of a lithium secondary battery move to an anode active material, such as a graphite-based material, to be intercalated into interlayers of the anode active material. In this case, since lithium is highly reactive, lithium reacts with an electrolyte and carbon constituting the anode active material on a surface of the anode active material, such as a graphite-based material, to form a compound such as Li2CO3, Li2O, or LiOH. These compounds may form a solid electrolyte interface (SEI) on the surface of the anode active material such as a graphite-based material.
The SEI may only pass lithium ions by acting as an ion tunnel. Due to the effect of the ion tunnel, the SEI may prevent the destruction of an anode structure due to the intercalation of organic solvent molecules having a high molecular weight, which move with lithium ions in the electrolyte, into the interlayers of the anode active material. Thus, the decomposition of the electrolyte does not occur by preventing the contact between the electrolyte and the anode active material, and stable charge and discharge may be maintained by reversibly maintaining the amount of lithium ions in the electrolyte.
Typically, with respect to an electrolyte solution which does not include an electrolyte solution additive or includes an electrolyte solution additive having poor characteristics, it is difficult to expect the improvement of lifetime characteristics due to the formation of a non-uniform SEI. Furthermore, even if an electrolyte solution additive is included, in the case that the addition amount thereof is not controlled to the required amount, the surface of a cathode may be decomposed or an oxidation reaction of the electrolyte may occur during a high temperature reaction due to the electrolyte solution additive. Thus, eventually, irreversible capacity of a secondary battery may increase and lifetime characteristics may degrade.